Delesse's Rule (thing)

Achille Ernest Delesse was a mid-nineteenth century miningengineer. He had an interest in determining the compostition of rocks. To find out how much of one paticular mineral a rock contained, he cut though it, polished an exposed face, and covered the face with transparent waxed paper, trimmed to size. He then traced the exposed portions of the mineral that interested him. He would weigh the paper, then cut out the traces of the minerals and weigh it again. The ratio of the weights gave the proportion of the mineral in the entire rock occupied by the mineral. The rule is still used by petroleumgeologists today.